The present invention is directed toward dishes and related wares for use to bake, brown and warm food in microwave ovens. The method of manufacture of such wares is also a feature of the invention. More particularly, the present invention relates to ferrite-containing material in combination with specific binders along with a method of applying same to fast moving metal strips resulting in a material which may be cold drawn without fracture to the coating then or after subjecting the material to microwave energy.
The heating of food articles with microwave energy by consumers has now become commonplace. Such microwave heating provides advantages of speed and convenience. However, certain food items, such as breaded fish, when heated with microwave energy often results in a soggy texture to the food and fails to impart the desirable browning flavor and crispness of conventional heating ovens, partly due to retention of oil and moisture and also due to the inherent nature of microwave heating.
When microwave ovens were first marketed for home kitchen use, the customary experience was for the food to warm and cook but not the container. Although the container often became warm, this was due to conduction of heat from the food and therefore the dish was limited to the heat of the food or some temperature less. This is still the situation today where "conventional" microwave cooking is employed utilizing cooking wares that are not heated by exposure to microwave energy but rather indirectly, by the food.
Notwithstanding the past and existing experiences, there has been a recognition that the dish can also serve as a cooking surface for the food and, therefore, items have been developed which heat when subjected to microwave energy. This discovery was based on the phenomenon that some materials will absorb microwave energy, converting it to heat and these materials are said to be lossy as contrasted with transparent materials through which microwave energy passes without generation of heat. By making a cooking ware of a lossy material, food can be cooked on the surface or exterior by conduction as well as by absorbing microwave energy. The use of lossy materials for browning or crisping is well known in the art.
Basically, there are three approaches to providing browning and crisping in a microwave environment. The first approach is to include an electrically resistive film usually quite thin for instance, 0.00001 to 0.00002 cms applied to the surface of a non-conductor or non-lossy substrate. In the case of a permanent dish as opposed to a disposable dish, the containers are frequently ceramic, and with a disposable package, the substrate can be a polyester film. Heat is produced because of the I.sup.2 R or resistive loss (see, for example U.S. Pat. Nos. 3,853,612, 3,922,452 and 3,783,220). Examples of disposable packing materials include metalized films such as those described in U.S. Pat. Nos. 4,594,492, 4,592,914, 4,590,349, 4,267,420 and 4,230,924.
The second category of microwave absorbing materials comprise electric conductors such as parallel rods or strips which function to produce an intense fringing electric field pattern which causes surface heating in an adjacent food. Examples include U.S. Pat. Nos. 3,271,552, 3,591,751, 3,857,009, 3,946,187 and 3,946,188. Such an approach is only taken with reusable utensils or dishes.
A third approach is to form articles from a mass or bed of particles that becomes hot in bulk when exposed to microwave energy. The microwave absorbing substance can be composed of ferrites, carbon particles, etc. Examples of such composition are articles prepared therefrom include U.S. Pat. Nos. 2,582,174, 2,830,162 and 4,190,757. These materials can readily experience run away heating and immediately go to temperatures in excess of 1200.degree. F. even with a food load to absorb the heat so generated. Some control over final heating temperatures is obtained by lowering so called Curie points of the materials by additions of dopants or selected binders. The Curie point is that temperature beyond which the material becomes transparent to microwave energy.
None of the prior art teaches how to formulate a material which can be adhered to a thin metal strip and thereafter cold drawn to provide an inexpensive and disposable food container which is capable in one embodiment, when subjected to microwave energy, of reaching food cooking temperatures of about 370.degree.-400.degree. F. in less than about 5 minutes and in another embodiment as a can for stew, soup or vegetables and the like which can reach food temperatures of about 120.degree.-140.degree. F. within about 3 minutes.